Properties of suspended sediment in the estuarine turbidity maximum of the highly turbid Humber Estuary system, UK

Measurements are presented of the properties of suspended particulate matter (SPM) in the estuarine turbidity maximum (ETM) of the upper Humber and Ouse estuaries during transient, relatively low freshwater inflow conditions of September 1995. Very high concentrations of near-bed SPM (more than 100 g l⁻¹) were observed in the low-salinity (less than 1), upper reaches. SPM within the ETM consisted largely of fine sediment (silt and clay) that existed as microfloc and macrofloc aggregates and individual particles. Primary sediment particles were very fine grained, and typically, about 20–30% was clay-sized at high water. The clay mineralogy was dominated by chlorite and illite. There was a pronounced increase in particle size in the tidal river, up-estuary of the ETM. The mean specific surface area (SSA) of near-bed SPM within the ETM was 22 m² g⁻¹ on a spring tide and 24 m² g⁻¹ on a neap tide. A tidal cycle of measurements within a near-bed, high concentration SPM layer during a very small neap tide gave a mean SSA of 26 m² g⁻¹. The percentage of silt and clay in surficial bed sediments along the main channel of the estuary varied strongly. The relatively low silt and clay percentage of surficial bed sediments (about 10–35%) within the ETM’s region of highest near-bed SPM concentrations and their low SSA values were in marked contrast to the overlying SPM. The loss on ignition (LOI) of near-bed SPM in the turbid reaches of the estuary was about 10%, compared with about 12% for surface SPM and more than 40% in the very low turbidity waters up-estuary of the ETM. Settling velocities of Humber–Ouse SPM, sampled in situ and measured using a settling column, maximized at 1.5 mm s⁻¹ and exhibited hindered settling at higher SPM concentrations.     

Effects of low dissolved oxygen events on the macrobenthos of the lower Chesapeake Bay

The effects of low dissolved oxygen or hypoxia (<2 mg l^?1) on macrobenthic infaunal community structure and composition in the lower Chesapeake Bay and its major tributaries, the Rappahannock, York, and James rivers are reported. Macrobenthic communities at hypoxia-affected stations were characterized by lower species diversity, lower biomass, a lower proportion of deep-dwelling biomass (deeper than 5 cm in the sediment), and changes in community composition. Higher dominance in density and biomass of opportunistic species (e.g., euryhaline annelids) and lower dominance of equilibrium species (e.g., long-lived bivalves and maldanid polychaetes) were observed at hypoxia-affected stations. Hypoxia-affected macrobenthic communities were found in the polyhaline deep western channel of the bay mainstem north of the Rappahannock River and in the mesohaline region of the lower Rappahannock River. No hypoxic effects on the infaunal macrobenthos were found in the York River, James River, or other deep-water channels of the lower Chesapeake Bay.     

Constraining Coronal Heating: Employing Bayesian Analysis Techniques to Improve the Determination of Solar Atmospheric Plasma Parameters

One way of revealing the nature of the coronal heating mechanism is by comparing simple theoretical one-dimensional hydrostatic loop models with observations at the temperature and/or density structure along these features. The most well-known method for dealing with comparisons like that is the χ ² approach. In this paper we consider the restrictions imposed by this approach and present an alternative way for making model comparisons using Bayesian statistics. In order to quantify our beliefs we use Bayes factors and information criteria such as AIC and BIC. Two datasets (Ugarte-Urra et al. 2005; Priest et al. 2000) are reanalyzed using the method described above. For the dataset of Ugarte-Urra et al. (2005), we conclude to apex dominant heating as the likely heating candidate, whereas the dataset of Priest et al. (2000) implies basal heating. Note that these new results are different from those obtained using the chi-squared statistic. For this we suggest that proper usage of Classical and Bayesian statistics should be applied in order to make safe assumptions about the nature of the coronal heating mechanisms.     

A review of experimental studies of crystal/liquid trace element partitioning

Available data on silicate/liquid, phosphate/liquid and oxide/liquid trace element partition coefficients from experimental studies show an encouraging degree of consistency, although much more work is required before a set of coefficients of wide-ranging application to planetary problems can be compiled. The complex dependences of most coefficients on bulk chemical composition (and liquid structure) are important aspects which remain to be fully resolved. Further determinations of coefficients for a number of elements for phosphates, zircon, spinels, amphiboles and garnets are especially desirable and investigations into the effects of pressure and volatiles on the magnitudes of partition coefficients are also needed. There is a major discrepancy among existing data regarding the upper concentration limits of Henry's Law dilute solution behavior under different experimental conditions. Data obtained at 1 atmosphere under dry conditions can apparently be reconciled with data from highpressure, H₂O-saturated experiments only if Henry's Law limits are themselves functions of variables such as pressure, temperature, H₂O activity and chemical composition. Further experiments (including studies bearing on the role of defect substitution at very low trace element concentrations) are required to resolve this question.     

Rainfall and irrigation controls on groundwater rise and salinity risk in the Ord River Irrigation Area,northern Australia

Groundwater beneath the Ord River Irrigation Area (ORIA) in northern Australia has risen in elevation by 10–20 m during the past 40 years with attendant concerns about water logging and soil salinization. Persistent groundwater accession has been attributed to excessive irrigation and surface water leakage; however, analysis of daily water-table records from the past 10 years yielded a contrary result. On a seasonal basis, water-table elevation typically fell during irrigation (dry) seasons and rose during fallow (wet) seasons, conflicting with the conventional view that irrigation and not rainfall must be the dominant control on groundwater accession. Previous investigations of unexpectedly large infiltration losses through the cracking clay soils provide a plausible explanation for the apparent conundrum. Because rainfall is uncontrolled and occurs independently of the soil moisture condition, there is greater opportunity for incipient ponding and rapid infiltration through preferred flow pathways. In contrast, irrigation is scheduled when needed and applications are stopped after soil wetting is achieved. Contemporary groundwater management in the ORIA is focused on improving irrigation efficiency during dry seasons but additional opportunities may exist to improve groundwater conditions and salinity risk through giving equal attention to the wet-season water balance.     

Cyanobacteria in New Zealand inland waters: Experimental studies

A small volume of experimental work with cyanobacteria in New Zealand has been produced through the last two decades. Except for some studies with geothermal organisms biased to New Zealand conditions, most work has followed the world‐wide trends in cyanobacterial research. There has been some emphasis on N₂‐fixing species, but symbiotic relationships, metabolic kinetics, nutrient and other environmental factors promoting ecological dominance, and toxicity have been investigated.     

The Association of Cobalt with Iron and Manganese (Oxyhydr)oxides in Marine Sediment

Formation and dissolution of authigenic Fe and Mn (oxyhydr)oxides influence cycling of trace metals in oxic/suboxic surface sediments. We used the diffusive gradients in thin films technique (DGT) to estimate the association of cobalt with iron and manganese oxides. We compared Co, Fe and Mn maxima measured by DGT in the pore waters of fresh and aged marine sediment cores and estimated the Co/Fe and Co/Mn ratios in the metal oxides. A Mn maximum was not visible in DGT concentration profiles of freshly collected sediment cores, but after ageing the sediment, we observed a distinct Mn peak, presumably due to broadening of the depth range over which the various electron acceptors occur. Estimated Co/Mn ratios from both experiments are within the range of literature values for marine sediments, but the value from the aged experiment is at the lower end of the range. This is attributed to stimulation of sulphate reduction and precipitation of cobalt sulphides. The good correlation between Co and Fe maxima in the fresh sediments is attributed to the similarity of their reactions with sulphide rather than Co being released during authigenic Fe oxide reduction.     

Expansion rate and geometry of floating vegetation mats on the margins of thermokarst lakes,northern Seward Peninsula,Alaska, USA

Investigations on the northern Seward Peninsula in Alaska identified zones of recent (<50 years) permafrost collapse that led to the formation of floating vegetation mats along thermokarst lake margins. The occurrence of floating vegetation mat features indicates rapid degradation of near‐surface permafrost and lake expansion. This paper reports on the recent expansion of these collapse features and their geometry is determined using geophysical and remote sensing measurements. The vegetation mats were observed to have an average thickness of 0.57 m and petrophysical modeling indicated that gas content of 1.5–5% enabled floatation above the lake surface. Furthermore, geophysical investigation provides evidence that the mats form by thaw and subsidence of the underlying permafrost rather than terrestrialization. The temperature of the water below a vegetation mat was observed to remain above freezing late in the winter. Analysis of satellite and aerial imagery indicates that these features have expanded at maximum rates of 1–2 m yr^‐1 over a 56 year period. Including the spatial coverage of floating ‘thermokarst mats’ increases estimates of lake area by as much as 4% in some lakes. Copyright © 2011 John Wiley & Sons, Ltd.